The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Most mature sandstone reservoirs produce crude oil along with an increasing volume of water. The percentage of water is defined as the Water Oil Ratio (WOR) for a given well. The water may either be what is known as good water, which displaces the crude oil out of the reservoir or bad water which is produced without contributing to the production of crude. Bad water is often the result of water invading the reservoir and communicating with the wellbore through permeable channels/fissures etc. As the water usually has a much lower viscosity than the crude oil at reservoir conditions, the effective permeability of the formation to water is much higher than to crude, which tends to result in more water than crude being produced once the water is in direct communication with the wellbore. Bad water production generally increases with time, as the water invades more of the reservoir and after stimulation treatments. The treatment either selectively stimulates the intervals with water due to differences in relative permeability (in the case of matrix treatment), or due to increased communication with the water bearing intervals in the case of a hydraulic fracturing.
In some cases it may be possible to decrease the production of water using a chemical or more commonly a polymer which provides a disproportionate permeability reduction with respect to water. These polymers are more commonly known as Relatively Permeability Modifiers (RPM) and decrease the effective permeability of the reservoir to water without decreasing the effective permeability to crude. An RPM is usually a low viscosity polymer which when injected into the matrix a) is highly charged and adheres to the surfaces in the pore spaces, and b) through a number of different mechanisms, such as swelling in the presence of water, reduces the size of the pore throats and hence the relative permeability of the matrix to water. The success of an RPM treatment depends among other things on distance from the sandface that the RPM can be placed; the greater the distance, the longer the effect of the RPM will last. RPMs have been used to treat both stimulated and un-stimulated reservoirs. In the case of stimulated reservoirs, RPMs have been included as part of both matrix and hydraulic fracturing treatments.